The present invention relates to a fine particle prepared by coating a surface of a porous composite oxide particle with a silica-based inorganic oxide layer. Also the present invention relates to a sol prepared by dispersing said fine particles in a dispersion medium and a production method for the same as well as to a substrate with an anti-reflection coating film containing said fine particles formed on the surface thereof.
The present inventors previously made an invention of silica sol as well as of composite oxide sol such as silica-alumina sol and silica-zirconia sol in which fine and porous particles each having a large specific surface area are dispersed (Japanese Patent Laid-Open Publication No. HEI 5-132309). The composite oxide sol is used by making use of its porosity for various applications such as a binder, an absorbent, a filler having low refractive index and others, in addition to its use as a catalyst.
Further to prevent reflection on a surface of a substrate such as glass or plastic sheet, there has been known the technology for forming an anti-reflection coating film on the surface, and for instance, a coating film made from a low refractive index substance such as a fluorine resin or fluoride magnesium is formed on a surface of a glass or plastic sheet by means of the coating method, the evaporation method, or the CVD method. These methods are, however, expensive in the cost, and the durability is not so high.
There has been known also the method in which an anti-reflection coating film having fine and uniform convex and concave sections is made from fine particles of silica by applying a coating liquid containing fine particles of silica on a surface of a glass sheet. In this method, the reflection is prevented by making use of the effect that normal reflection is reduced by random reflection of light on an irregular surface made from fine particles of silica, or by making use of an air layer in clearance between the fine particles, but it is difficult to fix the particles on the surface of the substrate or to form a single layer coating film, so that it is not easy to control the refractive index on the surface.
Further the present inventors have proposed a sol in which fine particles of composite oxide with low refractive index prepared by completely coating surfaces of porous fine particles with silica is dispersed (Refer to Japanese Patent Laid-Open Publication No. HEI 7-133105). To coat the fine particles completely, however, the step of coating the particles repeatedly or the step of subjecting the fine particles to heat processing is required, and further as a solvent such as water or alcohol is sealed in the coated particles, so that there is a limit in reduction of the refractive index. In addition, it has been found that a film or a resin plate obtained by dispersing the particles in resin or the like has weak adhesion between the resin and the particles, and further that also strength of the anti-refection film prepared with such the material as described above is rather low.
The present inventors disclosed the fact that silica particles each having an organic group directly bonded to silicon have high affinity with organic solvents or resins and are easily monodispersed in water. With the silica particles, particles hardly drop off from a mold prepared by mixing the silica particles in a resin, but it is impossible to completely prevent the particles from dropping off from the mold. Further particles with low refractive index, which can be used as a filler for adjusting a refractive index, have not been obtained.
Further even if a coating film is formed on a substrate such as glass or plastic sheet using a substance having a low refractive index such as silica fluorine resin or magnesium fluoride, when there is no mutual solubility (affinity) between the coating film made from the low refractive index substance and the substrate, the adhesiveness with the substrate is often low. In addition, the capability of preventing reflection may be insufficient according to used substrates.
It is an object of the present invention to provide a variety of fine particles having a structure wherein a porous composite oxide particle is coated with a porous silica-based inorganic oxide layer. It is also an object of the present invention to provide a sol with said fine particles, method for preparing the sol, and a substrate having a coating film comprising the fine particles thereon, which has a low refractive index and excellent in adhesion with a resin or the like, strength, the ability to reduce reflection and the like.
Fine particles according to the present invention are porous fine particles of composite oxide comprising silica and an inorganic oxide other than silica, wherein said fine particles are coated with a porous silica-based inorganic oxide layer having a thickness from 0.5 to 20 nm.
The maximum pore diameter of said silica-based inorganic oxide layer is preferably in the range from 0.5 to 5 nm.
The molar ratio (MOx/SiO2) is preferably in the range from 0.0001 to 0.2, when silica is expressed by SiO2 and the inorganic oxide other than silica is expressed by MOx.
The pore volume of said fine particles is preferably in the range from 0.1 to 1.5 cc/g and the particles further comprise a second silica coating film layer coated thereon preferably.
The fine particles include preferably an organic group directly bonded to silicon, and SR/ST, the ratio of the molar amount of silicon having the organic group directly bonded thereto (SR) vs the molar amount of the total silicon (ST), is preferably in the range from 0.001 to 0.9.
A method of producing fine particle dispersion sol according to the present invention comprises the steps of:
(a) preparing a dispersion liquid of core particle precursor by concurrently adding an aqueous solution of a silicate and/or an acidic silicate solution, a hydrolyte of an organic silicon compound expressed by the formula (1), and a solution of an alkali-soluble inorganic compound to an alkaline aqueous solution with the pH value of 10 or more, or to an alkaline aqueous solution with the pH value of 10 or more in which species particles are dispersed, if necessary;
(b) forming a first silica-based coating film layer on the core particle precursor by adding a silica source or an aqueous solution of the silica source and any inorganic salt other than silica to said dispersion liquid of core particle precursor; and
(c) selectively removing at least a portion of elements other than silicon and oxygen from among those constituting said core particle precursor by adding an acid to said dispersion liquid;
RnSiX(4xe2x88x92n)xe2x80x83xe2x80x83(1)
[wherein R indicates a non-substituted or substituted hydrocarbon comprising 1 to 10 carbon elements; X indicates an alkoxy group containing 1 to 4 carbon elements, a silanol group, halogen, or hydrogen, and n indicates a number from 1 to 3].
The producing method according to the present invention further comprises preferably the step of forming a second silica coating film layer on said fine particles by adding an alkaline aqueous solution, an organic silicon compound expressed by the formula (2) and/or a partial hydrolyte thereof to the fine particle dispersion sol prepared in the step (c).
RnSiX(4xe2x88x92n)xe2x80x83xe2x80x83(2)
[wherein R indicates a not-substituted or substituted hydrocarbon comprising 1 to 10 carbon elements; X indicates an alkoxy group containing 1 to 4 carbon elements, a silanol group, halogen, or hydrogen, and n indicates a number from 0 to 3]
Said method further comprises preferably the step of eliminating pores in the second silica coating film layer by heating the fine particle dispersion sol under the temperature from 50xc2x0 C. to 350xc2x0 C.
A substrate with a coating film according to the present invention is characterized in that said coating film contains the above-mentioned fine particles and a matrix for forming a coating film.
The refractive index of said substrate is preferably 1.60 or more.
A film-coated substrate preferably comprises an intermediate coating film with the refractive index of 1.60 or more formed on a surface of the substrate having the refractive index of 1.60 or less, said coating film containing the fine particles according to the present invention and a matrix for forming a coating film thereon. The intermediate coating film contains preferably fine particles of metal oxide with the average particle diameter in the range from 5 to 100 nm by 30 to 95 weight %.
Preferable embodiments of the present invention are described below.
A porous particle of composite oxide comprising silica and an inorganic oxide other than silica is used for a core of fine particle according to the present invention. The inorganic oxides available for the invention include one or more of Al2O3, B2O3, TiO2, ZrO2, SnO2, Ce2O3, P2O5, Sb2O3, MoO3, ZnO2, WO3. Inorganic oxides consisting of two or more compositions include TiO2xe2x80x94Al2O3 and TiO2xe2x80x94ZrO2.
The molar ratio MOx/SiO2 of the core particle should preferably be in the range from 0.05 to 2.0, when silica is expressed by SiO2 and an inorganic oxide other than silica is expressed by MOx. When the molar ratio MOx/SiO2 is less than 0.05, the pore volume described later does not become fully large with the refractive index reduced insufficiently, and on the other hand, when the molar ratio MOx/SiO2 is larger than 2.0, the obtained sol has low stability, which is not preferable.
A surface of the core particle is coated with a silica-based inorganic oxide layer. The silica-based inorganic oxide layer as defined herein includes (1) a silica monolayer, (2) a monolayer of a composite oxide comprising silyica and other inorganic oxide other than silica, and (3) a dual layer comprising the layer (1) and the layer (2) above.
In this invention, it is necessary to limit the thickness of the coating film in a range from 0.5 to 20 nm. When the thickness of the coating film is less than 0.5 nm, the coating effect can hardly be achieved, and more specifically a resin (including monomer, polymer, and oligomer; this is applied also in the following description), sometimes enters in the pores of the composite oxide particles and in that case it often occurs that lowering of the refractive index described below is insufficient. Further, in the production step described below, sometimes the particles may not preserve the original forms when at least a portion of the elements other than silicon and oxygen are selectively removed from the particles, which is not preferable. On the other hand, when the thickness of the coating film is over 20 nm, sometimes it becomes difficult to form a coating layer having the appropriate pore size as described below, and it may occur that the obtained particles are not porous. In addition, selective removal of elements in the next step is likely to become difficult. Further as a solvent such as water or alcohol is sealed in the composite oxide particles, when the particles are dispersed in a resin to form a coating film, the solvent can not sufficiently be removed in the drying step, and a gas phase such as air is not, formed, so that the refractive index can not sufficiently be lowered, which is not preferable. The thickness of the coating film should preferably be in the range from 1 to 8 nm.
The coating film is required to be porous, and the maximum value of the pore size should preferably be in the range from 0.5 to 5 nm. When the maximum pore size is less than 0.5 nm, the resin can hardly enter the fine pores of the coating film, so that adhesiveness of the coating film to the resin becomes insufficient. Further as the solvent in the fine pores of the composite oxide particles does not go out of the pores when the particles are dried, the refractive index lowers insufficiently, which is not preferable. On the other hand, when the maximum pore size is larger than 5 nm, as the resin enters not only the coating film layer, but also fine pores of the composite oxide particle as core particle, lowering of the refractive index is insufficient. The maximum fine pore size should more preferably be in the range from 1 to 4.5 nm.
The fine pore volume of the composite oxide particle should preferably be in the range from 0.1 to 1.5 ml/g, and more preferably be in the range from 0.2 to 1.5 ml/g. When the fine pore volume is less than 0.1 ml/g, porous fine particles having the desirable characteristics can not be obtained, old On the other hand, when the fine pore volume is more than 1.5 ml/g, strength of the fine particles becomes lower.
The average particle diameter of the composite oxide fine particles according to the present invention is required to be in the range from 5 to 300 nm. When the average particle diameter is less than 5 nm, the volumic percentage of the coating film layer in the fine particles increases, while a percentage of the fine pores decreases. On the other hand, when the average particle diameter is over 300 nm, it becomes difficult to obtain a stable dispersion sol, and further the transparency of the coating film containing the fine particles is apt to become lower. The average particle diameter of the fine particles of the composite oxide according to the present invention should preferably be in the range from 10 to 200 nm. Therefore the average particle diameter of the core particles constituting the fine particles of the composite oxide is required to be in the range from 4.5 to 280 nm. The average fine particle diameter can be obtained by the dynamic light scattering method.
The fine particle of composite fine particle according to the present invention should preferably include an organic group directly bonded to silicon. A substituted or non-substituted hydrocarbon group having 1 to 10 carbon elements is used as the organic groups as described above, and the hydrocarbon group includes a hydrocarbon group, a carbonized halogen group, an epoxy alkyl group, an amino-alkyl group, a methacrylalkyl group, and a melcaptoalkyl group. More specifically a methyl group, a phenyl group, an isobutyl group, a vinyl group, a trifluoropropyl group, a xcex2-(3,4 epoxy cyclohexyl) group, a xcex3-grycidoxy propyl group, a xcex3-methacryloxy propyl group, a N-xcex2(aminoethyl) xcex3-aminopropyl group, a xcex3-aminopropyl group, a N-phenyl-xcex3-aminopropyl group, a xcex3-mercaptopropyl group and the like can be enlisted as the hydrocarbon group described above.
SR/ST, the ratio of the molar amount of silicon having the organic group directly bonded thereto (SR) vs the molar amount of the total silicon (ST) should preferablybe in the range from 0.001 to 0.9. When the ratio SR/ST is less than 0.001, as the quantity of organic groups on the surface of the particle is too small, the affinity to the organic solvent or the resin is insufficient, and also lowering of the refractive index due to inclusion of the organic group is insufficient. On the other hand, when the ratio SR/ST is over 0.9, characteristics of the organic group is shown too strongly, and aggregation of the particles in water easily occur. The ratio SR/ST should preferably be in the range from 0.1 to 0.9.
The ratio SR/ST is calculated as described below. A sol is dried overnight in vacuum under the temperature of 100xc2x0 C., and about 5 g of powder sample obtained by removing evaporative components such as water is accurately weighted, and then is dispersed in 250 ml of 0.05 N NaOH aqueous solution, and the solution is agitated continuously for 10 hours under the room temperature. With this operation, all of the hydrolysable groups not reacted yet in the powder sample are hydrolyzed, and extracted in the water as the dispersion medium. The operations of separating the powder sample in said dispersion medium by means of ultracentrifugation and washing the separated powder sample with water, are repeatedly performed, and then the content of total carbon in the powder sample dried for 5 hours under 200xc2x0 C. is measured by means of the ultimate analysis, and the molar amount of silicon having the organic group directly bonded thereto (SR) is calculated from the average carbon numbers of the organic group used as the source, and finally the ratio to the molar amount of the total silicon (ST) is calculated.
The fine particle dispersion sol according to the present invention are dispersed in a dispersion medium such as water, an organic solvent, and a mixture solvent of water and any organic solvent. There is not specific limitation over the organic solvent to be used in this invention, and organic solvents used in the conventional types of organic sol including monovalent alcohol such as methanol and ethanol, and polyvalent alcohol such as ethylene glycol and propylene glycol may be used in this invention.
The sol described above can be used for various purposes, and when it is necessary to condense the sol, it is better to remove a portion of alkali metal ions, alkali earth metal ions, and ammonium ions previously, and then to condense the sol for obtaining more stable condensed sol. Any known method such as ultrafiltration may be used for removing the ions.
The method for preparing the sol according to the present invention comprises the steps (a) to (C) described above. The production method is described in detail below.
Step (a) [Preparation of a Dispersion Liquid in Which Precursors of Core Particles are Dispersed]
One or more silicates selected from the group consisting of an alkali metal silicate, an ammonium silicate, and a silicate of organic base are preferably used as a silicate. Sodium silicate (water glass) or potassium silicate can be enlisted as the alkali metal silicate; and a quarternary ammonium salt such as tetraethyl ammonium salt and amines such as monoethanol amine, diethanolamine, and triethanol amine as the organic base; and further an alkaline solution in which ammonia, quarternary ammonium hydroxide, or any amine compound is added in a silicic acid as the silicate of ammonium salt or organic base respectively.
As the acidic silicic acid solution, a silicic acid solution obtained by processing an alkaline silicic acid solution with a positive ion exchange resin to remove alkali may be used, and the acidic silicic acid solution with pH in the range from 2 to 4 and about 7 weight percent or less of SiO2 concentration is preferable.
As the organic silicon compound shown in the formula (1) above, there can be enlisted methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, diphenyldiethoxy-silane, isobutyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltris(xcex2-methoxyethoxy)silane, 3,3,3-trifluoropropyltrimethoxysilane, methyl-3,3,3-torifluoropropyldimethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex3-glycidoxytripropyltrimethoxysilane, xcex3-glycidoxyprbpylmethyldiethoxysilane, xcex3-glycidoxypropyltriethoxysilane, xcex3-methacryloxypropylmethyldimethoxysilane, xcex3-methacryloxypropyltrmimethoysilane, xcex3-methacryloxyprpylmethyidiethoxysilane, xcex3-methacryloxypropyltriethoxysilane, N-xcex2(aminoethyl)-xcex3-aminopropylmethyldimethoxysilane, N-xcex2(aminoethyl)-xcex3-aminopropyltrimethoxysilane, N-xcex2(aminoethyl)-xcex3-aminopropyltriethoxyisilane, xcex3-aminopropyltrimethoxysilane, xcex3-amino-propyltriethoxysilane, N-phenyl-xcex3-aminopropyltrimethoxysilane, xcex3-mercaptopropyltrimethoxysilane, trimethylsilanol, methyltrichlorosilane, methyldichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, phenyltrichlorosilane, diphenyldichlorosilane, vinyltrichlorosilane, trimethylbromosilane, and diethylsilane, or the like.
As the organic silicon compounds described above are not so hydrophilic, it is preferable to previously hydrolyze the compounds so that the compounds can homogeneously be mixed in the reaction system. For hydrolyzing the organic silicon compounds, any well-known method may be used. When a hydroxide of an alkali metal, ammonia water or amine which are all basic is used as a catalyst for the hydrolysis, after completion of the hydrolysis, the basic catalyst is removed, and the remaining acidic solution may be used. Further, when a hydrolyte is prepared by using an acidic catalyst such as an organic acid or an inorganic acid, it is preferable to remove the acidic catalyst, after completion of the hydrolysis, by means of, for instance, ion exchanging. It should be noted that the obtained hydrolyte of the organic silicon compound is used in the form of aqueous solution. The aqueous solution as defined herein indicates a state in which the hydrolyte is not gelled in the opaque state and it keeps the transparency.
It is preferable to use an alkali-soluble inorganic compound as a source for the inorganic oxide, and the inorganic compounds include, but not limited to an alkali metal salt, an alkali earth metal salt, an ammonium salt, a quarternary ammonium salt of the oxo acid of the above-described metals or non-metals, and more specifically sodium aluminate, sodium tetraborate, zirconyl carbonate ammonium, potassium antimonate, potassium stannate, sodiumaluminosilicate, sodiummolybdate, ceriumnitrate ammonium, and sodium phosphate.
To prepare a dispersion liquid of the core particle precursor, previously an alkaline aqueous solution of the inorganic compound is prepared discretely, or a mixture aqueous solution is prepared, and this aqueous solution is gradually added to an alkaline aqueous solution with pH 10 or more according to the target ratio of the composite oxide.
It is preferable to adjust addition rate of the silica source, organic silicon compound and inorganic compound to be added in the alkaline aqueous solution so that, when the silica component is expressed with SiO2 and the inorganic compounds other than silica with MOx, the molar ratio of MOx/SiO2 will be in the range from 0.05 to 2.0. When the molar ratio of MOx/SiO2 is less than 0.05, the fine pore volume described before is not sufficiently large, and on the other hand, when the molar ratio of MO2/ SiO2 is over 2.0, the stability of the sol obtained becomes lower. Further it is preferable to adjust the addition rate so that the molar ratio of SR/ST is in the range from 0.001 to 0.9.
Although the pH value of the solution changes simultaneously when the components are added to the solution, any specific operations for limiting the pH value in a prespecified range is not required in the present invention. The pH value of the aqueous solution is finally set to a particular pH value decided by the types and the mixing ratio of the organic silicon compound and the inorganic oxide. For controlling pH with in a prespecified range, sometimes an acid is added, but in this case, a salt of a metal as a source for the composite oxide is generated by the added acid, and because of this salt, sometimes the stability of the dispersion liquid dispersed core particles may drop. It should be noted that there is not any specific limitations over the addition rate of the aqueous solution.
In the production method according to the present invention, it is possible also to use a dispersion liquid of species particles as a starting material when preparing a dispersion liquid of core particles. In this case, fine particles of inorganic oxide such as SiO2, Al2O3, TiO2, ZrO2, SnO2 and CeO2, or of composite oxide of these inorganic oxides such as SiO2xe2x80x94Al2O3, TiO2xe2x80x94Al2O3, TiO2xe2x80x94ZrO2, SiO2xe2x80x94TiO2, SiO2xe2x80x94TiO2xe2x80x94Al2O3 are used as the species particles, and generally sols of these inorganic oxides or composite oxides may be used. The dispersion liquid of species particles can be prepared by any known methods in the art. The sol can be obtained, for instance, by adding an acid or an alkali to a metal salt corresponding to any of the inorganic oxides, a mixture of metal salts, or metal alkoxyde to hydrolyze the material and curing the hydrolyte according to the necessity. It is needless to say that the sol obtained by the production method according to the present invention may be used as a dispersion liquid of species particles.
The aqueous solution of the compound is added with agitation to the dispersion liquid of species particles with the pH having been adjusted to 10 or more, in the same way as that for adding it to the alkaline aqueous solution. In this case also, the pH control over the dispersion liquid is not performed and the PH is left in the uncontrolled state. When the composite oxide particles are grown from the species particles, it is easy to control the particle diameter of the grown particles, and particles with substantially uniform size can be obtained. The addition rate of the silica source, the organic silicon compound and the inorganic oxide to be added in the dispersion liquid of species particles should be in the same ranges as those in addition thereof to the alkaline aqueous solution.
The silica source, the organic silicon compound and the inorganic oxide source have high solubility in alkali side respectively. When two types of solutions are mixed in the pH area where the solubility of these compounds is high, the solubility of oxo acid ions such as, silicate ions and aluminate ions becomes lower, and these composite compounds are deposited and grow up to colloidal particles, or are deposited on the species particles, which leads to growth of the particles. Therefore, the pH control as required in the conventional technology is not always necessary in the deposition and the growth of the colloidal particles in this invention.
It is also possible to modify a surface of the particles by giving reactivity to an organic group to be introduced into the fine particles of composite oxide containing the target organic group and making the organic group react to a desired compound.
step (b) [Formation of First Silica-based Coating Film Layer]
As the silica source to be added, a silicate solution obtained by de-alkalizing an alkali metal salt of silica (water glass) is especially preferable. When the dispersion medium of the core particle precursor is water itself or the ratio of water against the organic solvent is high in the dispersion medium, it is possible to use the silicate solution for coating the particles. When a silicate solution is used, a specified quantity of silicate solution is added in the dispersion liquid, and at the same time alkali is added to have the silicate solution deposited on a surface of the core particles.
A hydrolysable organic silicon compound may also be used as the silica source. As the hydrolysable organic silicon compound, alkoxysilanes generally expressed by the formula RnSi(ORxe2x80x2)4xe2x88x92n [R, Rxe2x80x2: hydrocarbon groups such as alkyl group, allyl group, vinyl group, acryl group; n=0, 1, 2, or 3] may be used, and tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, and tetraisopropoxysilane are especially preferable.
When adding the silica source, a solution obtained by adding a small quantity of alkali or acid as a catalyst to a mixture solution of any of the alkoxysilanes, deionized water and alcohol, is added to a dispersion liquid of the core particle precursor to hydrolyze the alkoxysilanes, and the resultant silicate polymer is deposited on a surface of the precursor. In this step, the alkoxysilanes, alcohol and catalyst may be added at the same time to the dispersion liquid. As the alkaline catalyst, ammonia, hydroxides of alkali metal, or amines may be used. As the acidic catalyst, various types of inorganic acids or organic acids may be used. Further it is also possible to use both alkoxysilanes and the silicate solution for coating the particles.
Further for forming the first coating film layer comprising silica and inorganic oxide other than silica, in the step of forming the silica coating film layer, it is required only to add a source of any inorganic oxide other than silica. The inorganic oxide source may be added either before or after addition of the silica source, but it should preferably be added at the same time when the silica source is added. As for the addition ratio in this step, the molar ratio of the oxide other than that of silica against silica should preferably be in the range from 0.5 to 20.
When the molar ratio is less than 0.5, the pore volume and the pore diameter of the first coating film layer generated after a portion of the components other than silica is removed in the next step (c) are too small, and consequently the adhesiveness to the resin becomes insufficient. When the molar ratio is over 20, the pore diameter of the first coating film layer is too large, and the resin comes into and is solidified in the fine pores, so that the effect of lowering the refractive index may becomes smaller.
It should be noted that addition amounts of the silica source rand the source of the oxide other than silica must be sufficient to form a coating film layer with the thickness in the range from 0.5 to 20 nm. Further this first coating film layer may comprise a plurality of layers including the silica coating film layer and the coating film layer of the composite oxide comprising silica and any oxide(s) other than silica.
Step (C) [Selective Removal of Elements]
It is possible to increase pore volume of the core particle precursor forming the first coating film layer by selectively removing at least a portion of elements other than silicon and oxygen from those constituting the core particle precursor forming the first coating film layer.
To remove a portion of the elements other than silicon and oxygen, it is preferable to dissolve and remove the elements, for instance, by adding a mineral acid or an organic acid to the dispersion liquid of the fine composite oxide particles forming the silica-based coating film layer, or by contacting a positive ion exchange resin to the dispersion liquid for removing the elements by means of ion exchanging. After the elements are removed, the molar ratio of MOx/SiO2 should preferably be in the range from 0.0001 to 0.2.
The dispersion liquid from which some elements have been selectively removed can be washed by any known method such as the ultrafiltration. In this case, a sol in which fine particles are dispersed with high stability can be obtained by performing ultrafiltration after a portion of alkali metal ions, alkali earth metal ions, ammonium ions or the like in the dispersion liquid is removed beforehand. Further an organic solvent dispersion sol can be obtained by substituting the dispersion medium with any organic solvent according to the necessity.
In another production method of sol according to the present invention, further the step of forming a second silica coating film layer is added.
As the organic silicon compound expressed by the formula (2) above to be used in this step, any of the same organic silicon compounds as those used in the step (b) may be used. When an organic silicon compound with n of 0 in the formula (2) is used, the compound may be used as it is, but in an organic silicon compound with n of 1 to 3 in the same formula, it is preferable to use the same partially hydrolyzed organic silicon compound as that used in the step (a).
By forming the second silica coating film layer, it is possible to adjust thickness of the coating film layer, and it is possible to control the final thickness of the coating film layer within the range from 0.5 to 20 nm.
When the organic silicon compound with n of 1 to 3 in the formula (2) is used for forming the second silica coating film layer, it is possible to obtain a composite oxide fine particle dispersion sol with high dispersibility in an organic solvent and high affinity with a resin. Because of the characteristics, although generally surface processing with the silane coupling agent or the like is required, as the dispersibility in an organic solvent and affinity with a resin are excellent, the processing as described is not required in this case.
In still another production method of the fine particle dispersion sol according to the present invention, further a heat treatment step is added.
Namely, an alkaline aqueous solution is added to the dispersion sol of the fine particles of composite oxide containing the organic group with the second silica coating film formed thereon according to the necessary to control the pH of the dispersion sol preferably within the range from 8 to 13, and then the dispersion sol is subjected to heat treatment. The heat treatment in this step should preferably be performed under the temperature in the range from about 50xc2x0 C. to 350xc2x0 C. production and more preferably in the range from 100xc2x0 C. to 300xc2x0 C. By this heat treatment, it is possible to eliminate pores of the coating film layer without losing porosity of the core particles, and the sol in which sealed type of composite oxide fine particles with the core particles sealed with the silica-based coating film and dispersed can be obtained. In the heat treatment, the dispersion sol of fine particles of composite oxide containing the organic group obtained in the step (c) may previously be diluted or condensed for the heat treatment. Finally the dispersion sol subjected to the heat treatment may be washed like in the step (c).
The film-coated substrate according to the present invention is described below. This substrate is such a material as glass, polycarbonate, acrylic resin, plastic sheet such as PET or TAC, plastic film, or plastic panel, and has a coating film formed on a surface thereof, and can be obtained by applying a coating liquid described later by any of known methods such as the dip method, the spray method, the spinner method, or the roll coat method, and then drying and further sintering the coating film, if necessary.
The coating liquid described above is a mixture liquid of the sol and a matrix for forming a coating film, and sometimes an organic solvent may be mixed, if required.
The matrix for forming a coating film is a component capable of forming a coating film on a surface of the substrate, and such a material as a resin suited to the conditions such as adhesiveness to the substrate, hardness, and adaptability to being be applied and spread on a surface of the substrate can be selected for use, and materials available as the matrix include, but not limited to polyester resin, acrylic resin, urethane resin, vinyl chloride resin, epoxy resin, melamine resin, fluoride resin, silicone resin, butyral resin, phenol resin, vinyl acetate resin, ultraviolet curing resin, electron beam curing resin, emulsion resin, water-soluble resin, hydrophilic resin, each of which has been used in the art, or a mixture of these resins, and further copolymers or denatured ones of these resins, and hydrolysable organic silicon compounds such as the alkoxysilanes described above.
When a resin for coating is used as the matrix, for instance, an organic solvent dispersion sol in which water as the dispersion medium is substituted by an organic solvent such as alcohol, or an organic solvent prepared by treating the fine particles described above with any known coupling agent and then dispersing the treated fine particles in an organic solvent and a resin for coating are diluted with an appropriate organic solvent, and the resultant diluted mixture solution may be used as a liquid for coating.
On the other hand, when a hydrolysable organic silicon compound is used as the matrix, for instance, a partially hydrolysed alkoxysilane is obtained by adding water and an acid or an alkali as a catalyst to a mixture solution of alkoxysilane and alcohol, then the sol is mixed in the partially hydrolysed alkoxysilane with the mixture diluted with an organic solvent according to the necessity, and the resultant diluted mixture solution can be used as the coating liquid.
A weight ratio of the fine particles and the matrix in the coating liquid should preferably be in the range from 1/99 to 9/1. When the weight ratio is over 9/1, the strength of the coating film is insufficient and is not appropriate for practical use, and when the weight ratio is less than 1/99, there appear no effects even when the fine particles are added.
A refractive index of the coating film formed on the surface of the substrate as described above varies according to the mixing ratio of the fine particles and the resin or other components and the refractive index of the resin used for the purpose, but is generally low and in the range from 1.28 to 1.50. It should be noted that the refractive index of the fine particle itself according to the present invention is in the range from 1.20 to 1.44.
The fine particles according to the present invention have low refractive index, because even if dispersion medium enters pores of the fine particles, the dispersion medium goes out of the pores and the pores become empty when the coating film is dried, and because the pore diameter of the silica-based coating film layer is controlled within the range described above, so that components such as a resin used for forming the coating film are retained within the silica-based coating film layer, and then pores in the silica-based coating film layer are blocked after the resin is cured with porosity inside the particles maintained.
On the other hand, in the porous fine particles based on the conventional technology as described above (as disclosed in Japanese Patent Laid-Open Publication No. HEI 5-132309), components such as a resin used for forming a coating film enter to pores, the low refractive index as achieved with the present invention can not be obtained. Further the refractive index of the fine particles according to the present invention is lower even as compared to that of the fine particles with a solvent shielded in fine pores thereof obtained by coating the surface of the porous fine particles completely (as disclosed in Japanese Patent Laid-Open Publication No. HEI 7-133105).
In the film-coated substrate, when the refractive index of the substrate is less than 1.60, it is recommended to form a coating film with the refractive index of 1.60 or more (this coating film is sometimes described as intermediate coating film hereinafter) on the surface of the substrate and then form a coating film containing the fine particles according to the present invention thereon. When the refractive index of the intermediate coating film is over 1.60, the difference from the refractive index of the coating film containing the fine particles according to the present invention (which is sometimes described as surface coating film hereinafter) is large, so that a film-coated substrate with excellent anti-reflection capability can be obtained. The refractive index of the intermediate coating film can be adjusted by changing a type of fine particles of metal oxide to be used, a mixing ratio between the metal oxide and resin or other components, and a refractive index of a resin to be used.
A coating liquid for forming the intermediate film is a mixture solution of metal oxide particles and a matrix for forming a coating film thereon, and an organic solvent may be mixed according to the necessity. As the matrix for forming a coating film thereon, the same matrix as that for the coating film containing fine particles according to the present invention may be used, and by using the same matrix for forming a coating film, it is possible to obtain a film-coated substrate with excellent adhesiveness between the two types of coating films described above.
The weight ratio between the fine particles of metal oxide and the matrix in a coating liquid for forming the intermediate coating film (fine particles/matrix) should preferably be in the range from30/70 to 95/5, and more specifically from50/50 to 80/20. When the weight ratio is over 95/5, strength of the coating film is insufficient, and further the adhesiveness with the substrate is not sufficient so that the coating film is not suited to actual use, and on the other hand, when the weight ratio is less than 30/70, the refractive index of the intermediate film is not over 1.60, so that the difference from that of the surface coating film is not so large and its anti-reflection capability is not sufficient.
As the fine particles of metal oxide described above, it is preferable to use one with the refractive index of 1.60 or more, and more preferably with the refractive index of 1.70 or more, and the metal oxides which can be used for that purpose include, but not limited to, titanium oxide (2.50), zinc oxide (2.0), zirconium oxide (2.20), cerium oxide (2.2), tin oxide (2.0), thallium oxide (2.1), bariumtitanate (2.40), aluminum oxide (1.73), magnesium oxide (1.77), yttrium oxide (1.92), stibium oxide (2.0), and indium oxide (2.0). Of these oxides, conductive fine particles of titanium oxide, cerium oxide, tin oxide, stibium oxide, zirconium oxide, indium oxide or the like, and the conductive fine particles in which different types of elements such as stibium, tin, fluorine or the like are doped are preferable because the contained coating film has, in addition to the anti-reflection capability, the anti-electrification effect. When the refractive index of metal oxide fine particles is less than 1.6, the refractive index of the obtained intermediate coating film is not over 1.60, so that the difference in refractive index from the surface coating film is not so large and its anti-reflection capability is not sufficient.
The average diameter of the fine particles of metal oxide should preferably be in the range from 5 to 100 nm, and more preferably from 10 to 60 nm. It is impossible to obtain particles with the average particle diameter of less than 5 nm, and when the average particle diameter is over 100 nm, scattering of visible light becomes remarkable and transparency of the coating film becomes lower, which is not preferable.
When the coating liquid for forming the intermediate coating film is prepared with the fine particles of metal oxide described above, it is preferable to use the fine particles of metal oxide as a sol in which the fine particles are dispersed in a dispersion medium, and a water dispersion sol in which the fine particles are dispersed in water, an organic solvent dispersion sol in which the fine particles are dispersed in an organic solvent such as alcohol, and an organic solvent sol prepared by treating the fine particles with any known coupling agent and then dispersing the fine particles in an organic solvent and a resin for paint are diluted with an appropriate organic solvent to obtain a coating liquid. Further a surfactant may be added to the coating liquid for improving such characteristics as dispersibility and stability.
The two-layered film-coated substrate can be obtained by applying a coating liquid for forming an intermediate coating film, drying the coating film, forming the intermediate coating film according to the necessity, then applying a coating liquid for to form the surface coating film in the same way as that forming the intermediate film, drying the surface coating film, and sintering the substrate, if necessary. The two-layered film-coated substrate can also be obtained by applying the coating liquid for forming the intermediate coating film, drying the intermediate coating film, then applying the coating liquid for forming a surface coating film on the intermediate coating film, and then sintering the substrate, if necessary.